How To Can I leave this as the final finish?

[Fabtoy Knives]

I'm new to the forums and a relatively new bladesmith - the knife I'm talking about will number 22 that I've made and I have eight more at various stages of construction. I started knife-making almost two years ago doing stock removal.

On this knife, I used sodium bisulfate to dissolve forge scale following tempering, on a stainless blade. I then washed with baking soda and scrubbed with a steel brush. I was going to follow with sandpaper, but I noticed that this process left a very nice black patina on the blade. My question is, can I leave this as the final finish on my blade? Is it a permanent patina, or should I sand it off as I previously planned? Any serious and well-intentioned advice on this specific question is welcome.

 

[running bird]

The patina won’t be permanent, cutting abrasive material will scratch it. As long as you did most of your grinding pre HT and the edge is ground past any decarb (I don’t use stainless steels so I’m not familiar with how much you will have at the edge) then you can leave it how you want. You can use this one as a test knife and see how the finish does over time, and how it feels during cutting. depending on how porous the surface is from the finish I wonder if it will effect the corrosion resistance at all

 

[DevinT]

My guess is that the edge is too thick. Usually forging gets you close but you need to grind to the finished thickness.

Also, stainless requires a full anneal after forging. Otherwise, the structure will be very poor going into heat treating.

Lastly, you can’t heat treat stainless in a forge.

Hoss

 

[Fabtoy Knives]

The patina won’t be permanent, cutting abrasive material will scratch it. As long as you did most of your grinding pre HT and the edge is ground past any decarb (I don’t use stainless steels so I’m not familiar with how much you will have at the edge) then you can leave it how you want. You can use this one as a test knife and see how the finish does over time, and how it feels during cutting. depending on how porous the surface is from the finish I wonder if it will effect the corrosion resistance at all

Thanks!

 

[Fabtoy Knives]

My guess is that the edge is too thick. Usually forging gets you close but you need to grind to the finished thickness.

Also, stainless requires a full anneal after forging. Otherwise, the structure will be very poor going into heat treating.

Lastly, you can’t heat treat stainless in a forge.

Hoss

Thanks for the unrelated advice!

 

[DevinT]

Thanks for the unrelated advice!

It’s related, I’ve seen this movie before.

Hoss

 

[Stacy E. Apelt - Bladesmith]

Welcome Fatboy Knives. Fill out your profile so we know where you live and a bit about you.

Devin isn't giving "unrelated advice". He is pointing out what a lifetime of experience by him, thousands of other bladesmiths, and and modern science has taught him.
Devin is a Blade Cutlery Hall of Fame knifemaker and his son, Larrin, literally wrote the book on knifemaking metallurgy and HT (Knife Engineering). Devin started making knives before you were born. He became one of the leading suppliers of damascus in the industry. His presence and advice here is a welcome addition to the forum.

1) Stainless steel has to be hardened in a HT oven. An attempt to do it in a forge like a carbon steel will not work successfully. Stainless needs much higher temperatures than carbon steel (2000°F range vs 1500°F range), and much longer soak times (20-30 minutes vs 2-10min). Stainless blades need to be encased in a stainless foil packet to keep oxygen away from the blade. If your stainless blade had "forge scale" on it after HT that is a very bad sign. If not heated to at least 1900°F and held for at least 10 minutes, it probably didn't harden much at all.

2) The blade has a layer of decarb on it after HT. This needs to be sanded off. It is more than just sanding it down to a bright steel. The decarb is probably a few thousandths of and inch thick. A stainless blade done in a forge would have a huge layer of decarb (which you might have thought was decarb). Decarb doesn't dissolve in pickling with sodium bisulfate. It needs to be ground/sanded off completely to expose the hardened steel.

3) Any surface coloration after HT or after a chemical soak is merely a surface phenomenon. It will rub off or sand away quickly. The color difference you observe is also because of the decarb, which looks different than hardened steel.

4) There are other things, but the above should give you some areas of thought.

 

[NickBoyle]

Hoss has forgotten more about knife making than many of us will ever know, I don’t care if his advice is “unrelated” or not, if my question was “how do I forge (blah blah blah)” and he replied “this is how you skin the hair off a frogs ass….” I’d be headed to the pond looking for a frog just sayin

 

[running bird]

I hadn’t even noticed the forge part in the OP… that changes a lot.

Also you’ll want to listen to guys like Devin and Stacy. They’re here to help and have tons of advice and information to give.

My first post here years ago I knew absolutely nothing about steel or making knives. Before then I had made some kind of knife like objects but didn’t know anything beyond grinding metal I bought at the hardware store then gluing scrap wood onto it.

If I remember right, I’m pretty sure at the time I assumed you could add carbon to any metal and it would make it harden like steel. I posted asking about how to add carbon to copper, then fold and forge it so I could make it harden like steel. It made no sense at all and I didn’t understand what I was talking about. The guys here pointed me in the right direction…

I still don’t know anything but I know more than I did then. Asking questions here, listening and understanding the advice you’re given, then applying it the best you can is a better way to go about things. That will make the time you spend making knives a lot more worthwhile and meaningful.

 

[Drew Riley]

There's no such thing as a permanent patina. At least not on any knife that's going to be used.